Parser

The package org.apache.lucene.queryparser.flexible.parser contains interfaces
that should be implemented by the parsers.
Parsers produce QueryNode Trees from a string object.
These package still needs some work to add support to for multiple parsers.
Features that should be supported for the future, related with the parser:
- QueryNode tree should be able convertible to any parser syntax.
- The query syntax should support calling other parsers.
- QueryNode tree created by multiple parsers.